Present advanced and future power and electronic circuits require higher energy or power density, and/or higher temperature capacitors than those presently available. Applications include for example, inverter and pulse forming network circuits, electronic filters in mobile, or ground based as well as spaced based-systems. Polymer film dielectrics today provide the highest energy and power density capacitors available, in the range of 100-1000+ volts, although there are limitations. These capacitors contain windings of film and foil electrodes, and an impregnant for high voltage operation. This wound structure provides a more efficient capacitor in terms of weight and volume than stacked-layer capacitors. The use of ceramic materials for high energy and power density as well as high temperature operation is very attractive because ceramics can have both high dielectric constant (far higher than available polymeric films which display .epsilon..about.10 or less) and also high temperature stability. Ceramics can have dielectric constants (.epsilon.'s) ranging from the low single digits (e.g., SiO.sub.2 =3.8) to tens of thousands for complex multiphase materials. However, those having very high dielectric constant generally have significantly high dielectric losses (Tan .delta.), which, for some applications is not allowable. The more common ceramic materials used in capacitor manufacture have values .epsilon. in the low hundreds, with reasonably low dielectric losses, and also reasonable high dielectric strength (voltage stand-off). The table below lists some typical capacitor ceramic materials.
TABLE I ______________________________________ Dielectric properties for typical candidate for high power capacitors. Ceramic Dielectric constant .epsilon. Dielectric loss Tan .delta. ______________________________________ SiO.sub.2 3.8 0.0003 Al.sub.2 O.sub.3 10 0.0001 BaO 34 0.001 TiO.sub.2 90 0.0005 PbTiO.sub.3 110 0.003 BaTiO.sub.3 2000-4500 0.003 NPO * 62 &lt;0.002 Yb doped 500-3900 0.06-0.33 MgTiO.sub.3 14 0.0025 CaTiO.sub.3 153 0.0003 SrTiO.sub.3 240 0.00012 PZT (8/90/10).sup.(1) * 303 0.004 PMN89.sup.(2) * 6700-9000 0.003 ______________________________________ *Commercial material .sup.(1) LeadZirconate-Titanate .sup.(2) Lead MagnesiumNiobate
Ceramic capacitors manufactured by conventional procedures of blending powders and sintering them in pellet form contain numerous defects, typically in the form of porosity, which lowers their dielectric strength. Because of this, electrical strength of .about.10 V/.mu.m is typical, and thus ceramics are used only for low voltage stress applications.
Another method of fabricating ceramic capacitors is the fabrication of ceramic precursor "tape" which involves blending the ceramic powder with organic binders and other additives in a solvent system, and casting the resulting slurry onto a carrier film, such as Mylar.RTM., in order to form a flexible `green` tape after evaporation of the solvent, having a thickness controlled during the casting operation. These green tapes are metallized using a screen printing process wherein a thin layer of conductor metal, also in an organic carrier system, is applied to tape surface. Once the solvent has evaporated (and the tape is `dry`), the tape is cut or punched into smaller pieces, the pieces stacked, and laminated together using heat and pressure to form a multi-layer capacitor. Finally, the multi-layer capacitor is sintered to remove the organic materials and densify the ceramic into a high density monolithic piece. This approach provides high capacitance in a small volume.
The deficiency of this approach is that the quality of the tape and the final ceramic suffers from non-uniformity in the sintering of the ceramic powder. Because of the irregular shape of all normally processed ceramic powder particles, the packing of the powder in the green tape is not totally uniform, and during the sintering operation, voids are formed which lead to defects in one or more of the layers of the capacitor, and the overall ceramic capacitor does not possess the properties that a fully dense, pore free material should have. In particular, the voltage stand-off is poor. This problem has not been solved by the capacitor manufacturers.
The solution to this problem is an object of the present application. Not only does the invention provide a fully dense, defect free capacitor, but the dielectric constant of the ceramic can be varied at will, and the design of the capacitor is not limited to a multi-layer design; it can be a wound design.